The American Institute of Architects’ Architecture Billings Index (ABI) reflected a sizable and unexpected increase in demand for architects’ design services in December, the organization reported this week. The December ABI score was 55.9, compared with 50.9 in November. This score points to a fairly robust increase in design services (any score above 50 indicates an increase in billings).

In fact, December saw the highest monthly growth of the year and brought the average monthly reading for 2016 up to 51.3, almost equaling the 2015 average of 51.6. The ABI is a leading economic indicator of construction activity, reflecting the roughly nine- to 12-month lead time between architecture billings and construction spending, so construction should see a parallel rise next fall.

Survey participants are asked whether their billings increased, decreased, or stayed the same in the month that just ended as compared to the prior month, and the results are then compiled into the ABI. The index scores are centered around 50, with scores above 50 indicating an aggregate increase in billings, and scores below 50 pointing to a decline.

Separately, project inquiries and new design contracts both had positive readings in December, the AIA reported, but didn’t rise at nearly the same pace as billings. That would suggest that firm billings are likely to see moderate growth moving into 2017.